Decay heat generation associated with the accumulated inventory of radioactive fission products provides a potentially self-destructive energy source in nuclear reactor cores, and therefore requires a continuously available cooling capability, even following shutdown from normal operation. In reactors of the light-water-cooled type (LWR), such as the pressurized-water-reactor (PWR) and boiling-water-reactor (BWR), it is at least hypothetically possible that a loss-of-coolant accident (LOCA) could occur together with sumultaneous failure of emergency core cooling systems (ECCS). In this event the core 1, generally consisting of uranium dioxide fuel clad in zirconium alloy tubing, would melt, and in the absence of subsequent intervention could melt through the reactor vessel and then breach the primary system containment, permitting access of the highly radioactive debris to the uncontrolled environment.